A fuel cell is a power generation system which oxidizes a fuel by an electrochemical process to directly convert energy discharged by an oxidizing reaction into electric energy, and the system has a stack structure in which a plurality of membrane-electrode assemblies are stacked. In each assembly, both side surfaces of an electrolytic film for selectively transporting hydrogen ions are sandwiched between a pair of electrodes made of a porous material. Above all, a solid polymer electrolyte type fuel cell in which a solid polymer film is used as an electrolyte can easily be made compact at a low cost, and additionally it has a high output density. In consequence, the use application of the fuel cell as a car-mounted power source is expected.
In this type of fuel cell, a range of 70 to 80° C. is usually considered to be an optimum temperature range for power generation, but in an environment of a cold district or another place, a long time is sometimes required from a time when the fuel cell starts to a time when the optimum temperature range is reached. Hence, various types of warm-up systems have been investigated. For example, in Japanese Patent Application Laid-Open No. 2002-313388, a technique is disclosed which controls the self-heating amount of the fuel cell by operation at a low power generation efficiency as compared to a normal operation, and warms up the fuel cell while running a vehicle. In such a technique, the output voltage of the fuel cell is set to a voltage value lower than that based on I-V characteristics (current-voltage characteristics) of the fuel cell to increase the thermal loss of the fuel cell and perform a warm-up operation by the self-heating. Therefore, any device for the warm-up does not have to be mounted, and the technique is excellent in convenience.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-313388